Process for recycling product streams separated from a hydrocarbon-containing feed stream

ABSTRACT

The present invention relates to a process for recycling product streams that have been separated from a hydrocarbon-containing feed stream comprising olefin monomer, olefin co-monomer, hydrocarbon diluent and components such as H 2 , N 2 , O 2 , CO, CO 2 , and formaldehyde. In accordance with the present process a hydrocarbon-containing feed stream is separated into a) a first side stream comprising hydrocarbon diluent and olefin monomer; b) a second side stream which is substantially hydrogen-free and comprises hydrocarbon diluent and olefin monomer, c) a bottom stream comprising substantially olefin-free hydrocarbon diluent, and d) an overhead vapor stream comprising olefin monomer, hydrocarbon diluent and components such as formaldehyde, H 2 , N 2 , O 2 , CO and CO 2 . The present process further includes recycling said first and said second side streams in a polymerization process for preparing bimodal polyolefin.

FIELD OF THE INVENTION

The present invention relates generally to olefin polymerization. Inparticular, the present invention relates to a process for theseparation of a hydrocarbon-containing feed stream comprising olefinmonomer, one or more optional co-monomers and hydrocarbon diluent intodifferent product streams and includes recycling of the separatedproduct streams in a polymerization process for preparing bimodalpolyolefin. The present invention thus relates to an optimization of therecycle system and process during olefin polymerisation.

BACKGROUND OF THE INVENTION

Olefin polymerizations are frequently carried out using monomer, diluentand catalyst and optionally co-monomers and hydrogen in a reactor. Whenthe polymerization is performed under slurry conditions, the productconsists usually of solid particles and is in suspension in a diluent.The slurry contents of the reactor are circulated continuously with apump to maintain efficient suspension of the polymer solid particles inthe liquid diluent. The product is discharged by means of settling legs,which operate on a batch principle to recover the product. Settling inthe legs is used to increase the solids concentration of the slurryfinally recovered as product slurry.

Alternatively, the product slurry may be fed to a second reactorserially connected to the first reactor where a second polymer fractionmay be produced. Typically, when two reactors in series are employed inthis manner, the resultant polymer product is a bimodal polymer product,which comprises a first polymer fraction produced in the first reactorand a second polymer fraction produced in the second reactor, and has abimodal molecular weight distribution. The resultant product will alsousually consist of solid particles in suspension in a diluent and willthen be discharged from the second reactor using settling legs in asimilar way as explained above.

The product slurry recovered in an olefin polymerization processcomprises a slurry of polymer solids in a liquid that contains diluent,dissolved unreacted monomer, and optionally dissolved unreactedco-monomer. Typically this liquid also includes traces of heavierelements, e.g. oligomers, and lighter components including H₂, N₂, O₂,CO and/or CO₂. Catalyst will generally be contained in the polymer.

Once recovered from the reactor, the product slurry is discharged to aflash tank, through flash lines, where most of the diluent and unreactedmonomers and optionally unreacted co-monomers are flashed off.Afterwards, it is highly desirable to further treat the vapors in orderto recover the unreacted monomer, optionally unreacted co-monomer andthe diluent, since there is an economic interest in re-using theseseparated components including the monomer, co-monomer, and the diluent,in a polymerization process.

It is known in the art that a vaporous stream comprising unreactedmonomer, unreacted co-monomer and diluent issued from the effluent of apolymerization process may be treated in a distillation system forseparation of its components. Traditionally, the diluent is capturedthrough a complicated process so that such diluent can be recycled tothe reactor.

U.S. Pat. No. 4,589,957 for instance describes a separation process of ahydrocarbon-containing vaporous stream comprising monomer, co-monomerand diluent issued from the effluent of a homo-polymerization and/orco-polymerization process. The described process comprises subjectingthe vaporous stream to a two-stage distillation provided with a commonaccumulation zone wherein the condensate from the accumulation zoneserves as the source of feed for the second distillation and reflux forthe first distillation.

However, a problem encountered in many distillation systems, is thatthere is a sub-optimal separation of lighter components, including H₂,N₂, O₂, CO and/or CO₂, from recovered diluent. As a consequence, use ofseparated diluent streams containing these components in apolymerization process may seriously reduce polymerization efficiencyand result in sub-optimal polymerization conditions. Especially in thecase of re-using separated diluent streams in a polymerization processfor preparing bimodal polymer product, it is for instance required torecover diluent streams wherein the residual amount of lightercomponents such as hydrogen, is substantially reduced in order to beable to use these diluent streams in reactors wherein the highermolecular weight component of a bimodal polymer product is prepared.

An example of a recovery process that is currently applied to meet thisrequirement involves the production of large amounts of diluent streamsthat are substantially free of olefin. However, such recovery processinvolves the re-utilisation of a diluent stream which is in fact toopure for that purpose as it substantially lacks olefin monomer and hencealso is too costly for that use. Moreover, separation methods adapted torecover large amounts of substantially olefin-free diluent entail anumber of problems and disadvantages, including inter ails requiringhigh amounts of energy for carrying out the separation process;resulting in increased amounts of olefin monomers that have to beseparated from lighter components such as those given above; increasedloss of olefin monomer, reduced stability of distillation systems; etc.

In view of the above, there remains a great need in the art foroptimised methods for recycling hydrocarbon-containing feed streams thatneed to be separated into streams that can be recycled to apolymerization process, especially wherein bimodal polyolefins, such asfor instance bimodal polyethylene, is prepared. Furthermore, there is aneed in the art to provide a diluent recycle process that is lessexpensive to construct and/or to operate.

SUMMARY

The Applicants provide a process that overcomes at least some of theabove-mentioned problems. Thereto an optimised process for separating ahydrocarbon-containing feed stream into different product streams andfor re-using said separated product streams is provided. More inparticular, the herein provided process permits to optimally recycle theseparated streams in a polymerization process for preparing bimodalpolymer.

In a first aspect, the invention thereto provides a process forrecycling product streams separated from a hydrocarbon-containing feedstream comprising olefin monomer, one or more optional olefinco-monomer, hydrocarbon diluent and components such as H₂, N₂, O₂, CO,CO₂, and formaldehyde, wherein said hydrocarbon-containing feed streamis separated by the steps of:

-   -   a) introducing said feed stream into a first distillation column        for subjecting said feed to distillation conditions adapted to        remove        -   a1) a bottom stream comprising hydrocarbon diluent and one            or more optional co-monomer, and        -   a2) an overhead stream comprising hydrocarbon diluent,            olefin monomer and components such as H₂, N₂, O₂, CO, CO₂,            and formaldehyde;    -   b) condensing the overhead stream issued from the first        distillation column in step a2) to form a condensate and storing        said condensate in a separator (108) adapted to separate a vapor        stream and a liquid stream;    -   c) removing from said separator said vapor stream comprising        olefin monomer, hydrocarbon diluent and components such as        formaldehyde, H₂, N₂, O₂, CO and CO₂;    -   d) condensing the vapor stream removed in step c) to form a        condensate and storing said condensate in a separator adapted to        separate a vapor stream and a liquid stream (15);    -   e) removing from said separator said liquid stream of step d);    -   f) separating said liquid stream into a first side stream        comprising hydrocarbon diluent and olefin monomer; and a        remainder stream;    -   g) introducing said remainder stream in a second distillation        column and subjecting said remainder stream to distillation        conditions adapted to remove        -   g1) a bottom stream comprising substantially olefin-free            hydrocarbon diluent,        -   g2) a substantially hydrogen-free second side stream            comprising hydrocarbon diluent and olefin monomer, and        -   g3) an overhead vapor stream comprising olefin monomer,            hydrocarbon diluent and components such as formaldehyde, H₂,            N₂, O₂, CO and CO₂.

In accordance with the present method, it is further noted that vaporstream issued from the separator of the first distillation column issent/fed to the overhead condenser of the second distillation column.The present invention is thus characterized in that it comprises atleast two condensation/separation cycles provided in series. Thisadvantageously permits to limit monomer loss, and therefore to reduceproduction costs. In particular, in the case of polyethylene production,such mode of operation allows to limit the loss of ethylene with theethane purge in case of bimodal configuration of the reactors. Inanother embodiment of the present process, a portion of the condensatestored in step b) is removed as liquid stream and passed as reflux tothe first distillation column.

In a preferred embodiment, the invention provides a process wherein saidfirst and said second side streams are recycled in a polymerizationprocess for preparing bimodal polyolefin comprising at least twodifferent polyolefin fractions that have been obtained in two differentpolymerisation reactors connected to each other in series, and whereinone of said fractions has a higher molecular weight than said otherfraction, and wherein said second side stream is re-used in thepolymerization process wherein the polyolefin fraction having the highermolecular weight is prepared, and wherein said first side stream isre-used in the polymerization process wherein the other polyolefinfraction is prepared.

In other words, the present process involves the steps of recycling saidfirst and said second side streams in a polymerization process forpreparing bimodal polyolefin comprising at least two differentpolyolefin fractions that have been obtained in two differentpolymerisation reactors connected to each other in series, and whereinone of said fractions has a higher molecular weight than said otherfraction. The second side stream is re-used in the polymerizationprocess wherein the polyolefin fraction having the higher molecularweight is prepared and the first side stream is re-used in thepolymerization process wherein the other polyolefin fraction, i.e. thepolyolefin fraction having the lower molecular weight, is prepared. Thefirst side stream can thus be fed to a reactor in which the polyolefinfraction having the higher molecular weight is prepared, while thesecond side stream can be fed to the reactor in which said otherpolyolefin fraction is prepared.

In yet another embodiment, the invention provides a process in whichsaid bottom stream of step g1) is re-used in a polymerization processfor preparing bimodal polyolefin comprising at least two differentpolyolefin fractions that have been obtained in two differentpolymerisation reactors connected to each other in series, and whereinone of said fractions has a higher molecular weight than said otherfraction, and wherein said bottom stream is re-used in thepolymerization process wherein the polyolefin fraction having the highermolecular weight is prepared.

In another embodiment, the invention provides a process comprising thesteps of

-   -   h) condensing the overhead vapor stream obtained in step g3        optionally in admixture of the vapor stream removed in step c)        to form a condensate, and storing the condensate thus formed in        a separator; and    -   i) subjecting the stored condensate obtained in step h) to        steps e) to g).

Preferably, the process further comprises the step of removing from thecondensate stored in step d) a vapor stream comprising olefin monomer,and components such as formaldehyde, H₂, N₂, O₂, CO and CO₂; andrecovering olefin monomer from said vapor stream.

In yet another embodiment, the present invention provides a processwhich may further comprise the step of introducing the bottom stream ofstep a1) in a third distillation column (3) for subjecting said bottomstream to distillation conditions adapted to remove 1) a side streamcomprising one or more optional co-monomer, 2) an overhead streamcomprising hydrocarbon diluent and optionally co-monomer, and 3) abottom stream comprising heavy components.

Preferably the side stream 1) is taken from the lower part of thedistillation column e.g. from tray 3 of the column, when counting fromthe bottom of the column. Generally, the overhead stream of the thirddistillation column will contain only minor amounts of co-monomer.

This overhead stream of the third distillation column comprisinghydrocarbon diluent can be fed back to the first distillation column.Preferably, the overhead stream which exits the top of the thirddistillation column is first cooled down in an overhead condenser of thethird distillation column. Then the condensed stream, issued at theoutlet of the condenser of the third distillation column is collected ina reflux drum of the third distillation column. The condensate can thenbe split up in two parts: a first part thereof is sent as reflux to thethird distillation column and a second part is recycled as feed to thefirst distillation column.

Alternatively or in combination therewith, bottom stream of step a1) canalso advantageously be re-used in the polymerization process forpreparing bimodal polyolefin. Especially it can be fed to the reactor inwhich the polyolefin fraction with the higher molecular weight fractionis prepared. In bimodal configuration, the reactor in which thepolyolefin fraction with the higher molecular weight fraction isprepared is also the one in which co-monomer (e.g. hexene) concentrationis the highest.

It shall be noted that all values that are given herein in ppm are meantto refer to values of ppm by weight. Hence the terms “ppm” and “ppm byweight” are used herein as synonyms.

In another embodiment, the present invention provides a process whereinsaid first side stream of step f) comprises at least 3 wt %, morepreferably at least 5 wt % olefin monomer. In another embodiment, thepresent invention provides a process wherein said first side stream ofstep f) comprises less than 10 wt % olefin monomer, and for instanceless than 8 wt % olefin monomer.

In another embodiment, the present invention provides a process whereinsaid first side stream of step f) comprises between 100 and 10000 ppm byweight hydrogen, preferably between 100 to 5000 ppm by weight. In apreferred embodiment said first side stream of step f) comprises lessthan 500 ppm by weight hydrogen.

In still another embodiment, the invention provides a process whereinsaid second side stream of step g2) is removed from the upper third ofsaid second distillation column. In an embodiment, the present inventionprovides a process wherein said second side stream of step g2) comprisesat least 2 wt % olefin monomer, preferably at least 2.5 wt %, and morepreferably at least 3 wt % olefin monomer. In another embodiment, thepresent invention provides a process wherein said second side stream ofstep g2) comprises at most 5 ppm by weight, preferably at most 2 ppm byweight, more preferably at most 1 ppm by weight hydrogen.

In yet another embodiment, the present invention provides a processwherein said bottom stream of step g1) comprises less than 5 ppm byweight of olefin monomer, and preferably less than 1 ppm by weight ofolefin monomer. In another embodiment, the present invention provides aprocess wherein said bottom stream of step g1) comprises less than 5000ppm by weight of olefin co-monomer.

In another preferred embodiment, a process is provided wherein saidhydrocarbon-containing feed stream comprising olefin monomer, co-monomerand hydrocarbon diluent is an effluent stream obtained from apolymerization process for preparing monomodal or bimodal polyolefin.Preferably, said olefin monomer is ethylene, said co-monomer is 1-hexeneand said hydrocarbon diluent is isobutane.

The present process includes the recovery of a side stream which issubstantially free of hydrogen. This side stream is as rich as possiblein olefin monomer while still poor enough in hydrogen, so that it can befed to the reactor wherein the polymer fraction having the highermolecular weight is prepared during a bimodal polymerisation process.Consequently, the need for using an olefin-free hydrocarbon stream forthat same purpose is strongly reduced. Since the flow rate of steam usedfor reboiling the distillation column is directly proportional to theflow rate of olefin-free hydrocarbon stream to be obtained as bottomstream, reducing this bottom stream further makes it possible tosignificantly reduce the steam consumption necessary to ensure properre-boiling of the distillation column.

Another advantage of the present invention is that said substantiallyhydrogen-free side stream will take up olefin monomer, and hencesignificantly reduce—e.g. by more than 50%—the incondensable vaporousstream which contains the main part of the hydrogen entering the recyclesection and which is removed from the recycle section and sent to amonomer recovery unit. Hence, the present invention permits to recover alarger portion of the olefin monomer entering the recycle section beforeit is sent to a recovery unit compared to currently applied recoveryprocesses. For instance, the present invention permits to recover alarger portion of ethylene monomer entering the recycle section beforeit is sent to an ethylene recovery unit (ERU) compared to currentlyapplied recovery processes. In accordance with the present process,recovering a substantially hydrogen-free side stream from thehydrocarbon feed stream permits to significantly reduce the loss ofethylene monomer.

Further in accordance with the present process less incondensablevaporous stream containing lighter components needs to be sent to themonomer recovery unit and the size of the monomer recovery unit, forinstance an ethylene recovery unit, can be significantly reduced. Thishas important economic and environmental repercussions and benefits.

Furthermore, unexpectedly, the Applicants have seen that the presentprocess involving the recovery of a) a first side stream comprisinghydrocarbon diluent and olefin monomer; b) a second side stream which issubstantially hydrogen-free and comprises hydrocarbon diluent and olefinmonomer, c) a bottom stream comprising substantially olefin-freehydrocarbon diluent, and d) an overhead vapor stream comprisingremaining olefin monomer, remaining hydrocarbon diluent and remainingcomponents such as formaldehyde, H₂, N₂, O₂, CO and CO₂ has improvedefficiency and stability compared to conventional distillation systemwhich lack recovery of said second side stream.

The present optimized recovery process is particularly suitable forproviding diluents streams for re-use in a polymerization system forpreparing bimodal polymer product. In particular, the present inventionprovides a process enabling to separately recover I) a substantiallyhydrogen-free diluent side stream that can be used in the reactorwherein the higher molecular weight fraction of a bimodal polymer isprepared; and II) a diluent side stream that can be used in the reactorwherein the lower molecular weight fraction of a bimodal polymer isprepared. Thus the present process optimally provides two side streamsthat each can be re-used to feed diluent to the respective reactorsapplied in the polymerization process for preparing bimodal polyolefins,for instance bimodal polyethylene.

In another aspect, the invention therefore also relates to the use of aprocess according to the invention in a polymerization process forpreparing bimodal polyolefin, such as for instance bimodal polyethylene,comprising at least two different polyolefin fractions that have beenobtained in two different polymerisation reactors connected to eachother in series, and wherein one of said fractions has a highermolecular weight, comprising the steps of:

-   -   feeding olefin monomer, a diluent, at least one polymerization        catalyst, optionally hydrogen, and one or more optional olefin        co-monomer(s) to a first reactor,    -   polymerizing said olefin monomer in said first reactor to        produce a polymer slurry comprising a first polyolefin fraction        in the diluent,    -   transferring said polymer slurry from said first reactor to a        second reactor,    -   feeding olefin monomer, a diluent, optionally hydrogen, and one        or more optional olefin co-monomer(s) to said second reactor,    -   polymerizing said olefin monomer and said one or more optional        olefin co-monomer(s) in said second reactor to produce a slurry        comprising a second polyolefin fraction in the diluent, said        second polyolefin fraction having a different molecular weight        than the polyolefin fraction produced in said first reactor, and    -   discharging from said second reactor a slurry comprising bimodal        polyolefin in said diluent,    -   recovering bimodal polyolefin from the slurry by separating at        least a majority of the diluent from the slurry in a        hydrocarbon-containing feed stream, and    -   subjecting said hydrocarbon-containing feed stream to a process        as described herein.

In yet another aspect, the invention also provides a polymerizationsystem for preparing bimodal polyolefin comprising two polymerisationreactors connected to each other in series operably connected to adistillation system, the distillation system comprising a firstdistillation column and a second distillation column which are operablyconnected to each other in series; and

wherein said first distillation column which is configured to separate ahydrocarbon-containing feed stream comprising olefin monomer, optionallyone or more co-monomer and hydrocarbon diluent, into

-   -   a bottom stream comprising hydrocarbon diluent and one or more        optional co-monomer, and    -   an overhead stream comprising hydrocarbon diluent, olefin        monomer and components such as H₂, N₂, O₂, CO, CO₂, and        formaldehyde;        is provided with at least one condenser for condensing said        overhead feed stream to form a condensed stream and at least one        separator, operably connected to said condenser and adapted to        separate said condensed stream in a vapor stream and a liquid        stream; and        wherein said second distillation column which is configured to        separate a hydrocarbon-containing feed stream comprising olefin        monomer, optionally one or more co-monomer and hydrocarbon        diluent, into    -   a bottom stream comprising substantially olefin-free hydrocarbon        diluent,    -   a substantially hydrogen-free side stream comprising hydrocarbon        diluent and olefin monomer, and    -   an overhead stream comprising olefin monomer, hydrocarbon        diluent and components such as formaldehyde, H₂, N₂, O₂, CO and        CO₂,        is provided with at least one condenser for condensing said        overhead stream to form a condensed stream and at least one        separator, operably connected to said condenser and adapted to        separate said condensed stream in a vapor stream and a liquid        stream, of which a part is separated as a first side stream        comprising hydrocarbon diluent and olefin monomer.

The present invention will be further disclosed in detail hereunder. Thedescription is only given by way of example and does not limit theinvention. The reference numbers relate to the hereto-annexed figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents a schematic view of an embodiment of a distillationsystem comprising two distillation columns according to the presentinvention.

FIG. 2 represents a schematic view of an embodiment of a distillationsystem comprising three distillation columns according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a process for separating ahydrocarbon-containing feed stream comprising olefin monomer, one ormore optional olefin co-monomers, hydrocarbon diluent and componentssuch as H₂, N₂, O₂, CO, CO₂, and formaldehyde in different productstreams and further comprises recycling the separately recovered productstreams in a polymerization process, preferably for preparing bimodalpolymer. The present process is optimized in terms of separation intodifferent product streams and in terms of recycling of the separatedstreams, in particular by controlling or re-distributing the amounts ofrecovered product streams and the concentrations of reactants withinsaid recovered streams.

The hydrocarbon-containing feed stream that is separated according tothe present invention will generally be an overhead stream coming from aflash tank and purge columns of a polymerization reactor, wherein astream containing solvent/diluent, polymer and unreacted monomers isflashed or otherwise treated to remove solvent or diluent and monomersthere from.

In another embodiment said hydrocarbon-containing feed stream can be anoverhead stream coming from another distillation column.

The hydrocarbon-containing feed stream that is separated and recycledaccording to the present invention can be obtained from anypolymerization process producing an effluent comprising a slurry ofparticulate polymer solids suspended in a liquid medium comprising adiluent and unreacted monomer. Such reaction processes include thosewhich are known in the art as particle form polymerizations, alsoreferred to as slurry polymerization.

As used herein, the term “polymerization slurry” or “polymer slurry” or“slurry” means substantially a two-phase composition including polymersolids and liquid. The solids include catalyst and a polymerized olefin,such as polyethylene. The liquids include an inert diluent, such asisobutane, with dissolved monomer such as ethylene, optionallyco-monomer, such as 1-hexene, molecular weight control agents, such ashydrogen, antistatic agents, antifouling agents, scavengers, and otherprocess additives.

In a preferred embodiment, the present invention is directed to theseparation process of a vaporous stream, which is issued from theeffluent of an ethylene polymerization reaction. Suitable “ethylenepolymerization” includes but is not limited to homo-polymerization ofethylene, and co-polymerization of ethylene and a higher 1-olefinco-monomer such as butene, 1-pentene, 1-hexene, 1-octene or 1-decene. Apresently preferred component stream separated according to theinvention comprises monomer, such as ethylene, co-monomer, such as1-hexene, and diluent, such as isobutane. It should be recognizedhowever, that the distillation system of the invention is equallyapplicable to other monomer, co-monomer and diluent systems so long asfeed vapors comprise hydrocarbons, which permit separation bydistillation. Traces of both heavy components, e.g. oligomers, andlighter components such as formaldehyde, N₂, H₂, and components such asO₂, CO and CO₂ are generally also present in such effluent streams.

More in particular, the present invention relates to a separationprocess of a hydrocarbon-containing feed, wherein saidhydrocarbon-containing feed stream is an effluent stream obtained from apolymerization process for preparing monomodal or bimodal polyolefins,such as monomodal or bimodal polyethylene (PE), and preferably forpreparing bimodal polyethylene. “Bimodal PE” refers to PE that ismanufactured using two reactors, which are connected to each other inseries, the operating conditions being different in the two reactors.“Monomodal PE” is produced in a single reactor or using two reactors inseries, with identical operating conditions.

The term “separation” as used herein refer to the step of fractionatinga hydrocarbon-containing feed stream in different fractions, which canthan be re-used.

Further, in accordance with the present process the product streamsseparated from said hydrocarbon-containing feed stream are recycled inan olefin polymerization process, preferably in an ethylenepolymerization process, and more preferably in a polymerization processfor preparing bimodal polyolefins, such as bimodal polyethylene.

The terms “bimodal polyolefin product” or “bimodal polyolefin” as usedin the present invention are meant to designate polymer productscomprising at least two fractions of olefin polymer wherein one fractionhas a lower molecular weight than the other fraction. The terms “bimodalpolyethylene product” or “bimodal polyethylene” as used in the presentinvention are meant to designate polymer products comprising at leasttwo fractions of ethylene polymer wherein one fraction has a lowermolecular weight than the other fraction. Bimodal polyolefins, such asbimodal PE can be produced in a sequential step process, utilizingpolymerization reactors coupled in series and using different conditionsin each reactor, the different fractions produced in the differentreactors will each have their own molecular weight.

In an embodiment a polymerization process for preparing bimodalpolyolefin, such as bimodal polyethylene, is carried out in a doubleloop polymerization reactors unit consisting of two liquid full loopreactors, comprising a first and a second reactor connected in series byone or more settling legs of the first reactor connected for dischargeof slurry from the first reactor to said second reactor. The firstpolyolefin fraction, diluent and catalyst can be continuously ordiscontinuously transferred from said first reactor to said secondreactor.

More in particular, in an embodiment, a first polyolefin, e.g. a firstpolyethylene fraction is obtained by a first polymerisation process ofolefin monomer, such as e.g. ethylene, in a diluent, such as e.g.isobutane, in the presence of a catalyst. Such first polymerisationprocess comprises the steps of feeding olefin monomer, a diluent, atleast one polymerization catalyst, optionally hydrogen, and one or moreoptional olefin co-monomer(s), such as e.g. 1-hexene, to said firstreactor, and polymerizing said olefin monomer in said first reactor toproduce a first polyolefin fraction, diluent and catalyst is transferredfrom said first reactor to a second reactor. In the second reactor asecond polyolefin fraction is obtained by feeding olefin monomer, suchas e.g. ethylene, a diluent, such as e.g. isobutane, optionallyhydrogen, and one or more optional olefin co-monomer(s), such as e.g.1-hexene, to said second reactor; polymerizing said monomer and said oneor more optional olefin co-monomer(s) in said second reactor to producea second polyolefin fraction in said second reactor. Said secondpolyolefin fraction has a different molecular weight than the polyolefinfraction produced in said first reactor. From the second reactor bimodalpolyolefin product comprising said first and said second polyolefinfraction is then recovered. This bimodal polyolefin product is thensupplied, optionally in combination with one or more additives to anextruder.

In a particularly preferred embodiment of the above method, said secondpolyolefin fraction produced in said second reactor has a lowermolecular weight than said first polyolefin fraction produced in saidfirst reactor. In another preferred embodiment, hydrogen is added to thesecond reactor wherein the second polyolefin fraction is produced havinga lower molecular weight than said first polyolefin fraction.

In a preferred embodiment, a first polyethylene fraction prepared in afirst reaction is a high-molecular-weight (HMW) component, composed ofan ethylene homopolymer or copolymer, for instance with a weight-averagemolar mass 300,000 g/mol, preferably from 300,000 to 700,000 g/mol andvery particularly preferably from 300,000 to 600,000 g/mol, andpreferably having a higher molecular weight than the second polyethylenefraction. In another preferred embodiment, a second polyethylenefraction prepared in a second reaction is a low-molecular-weight (LMW)component, composed of an ethylene homopolymer or ethylene copolymer,for instance with a weight-average molar mass of from 8000 to 80,000g/mol, preferably from 20,000 to 70,000 g/mol and very particularlypreferably from 30,000 to 60,000 g/mol, and preferably having a lowermolecular weight than the first polyethylene fraction.

The term “recycling” or “re-use” are used in the present inventionherein as synonyms and both refer to the step of sending or feeding aproduct stream that has been separated from a hydrocarbon-containingeffluent stream back to a polymerization reactor for use therein.

Separation of a hydrocarbon-containing feed stream—also denotedhydrocarbon-containing effluent stream—into different separated productstreams is carried in a distillation system. The terms “distillationsystem” or “separation system”, “recovery system” or “recycle section”,are used in some embodiments of the present invention as synonyms andrefer to systems comprising all necessary equipment adapted to separateand recover unreacted reactants from the effluent stream of apolymerization reaction. Such recovery systems generally include one ormore distillation columns. The term “distillation zone”, “separationcolumn” and “distillation column” may be used herein as synonyms. In apreferred embodiment, the present distillation process is carried out ina distillation system, which comprises one or more distillation columns,e.g. two or three distillation columns.

In a preferred embodiment, one or more of said distillation columns aretray columns. Such tray columns comprise a number of trays of variousdesigns to hold up the liquid in order to provide better contact betweenvapor and liquid. Trays essentially act as a unit operation, eachaccomplishing a fraction of the separation between liquid and gas. It isclear that the more trays there are, the better the degree ofseparation, and thus the better column performance will be. However,using a large number of trays in distillation columns has importantdisadvantages, especially with regard to construction. Suitabledistillation systems comprise distillation system having column(s) witha low number of trays, preferably lower than 25, even more preferredlower than 20. Nevertheless, although distillation columns with a lownumber of trays can be used in the present process, improvements on theoperation of the present distillation systems, as explained in moredetail below, permit to achieve a similar degree of separation as withcolumns with a higher number of trays. Advantageously, application ofthe present process includes the benefits of less energy usage and lowerconstruction costs.

In an alternative embodiment, one or more of said distillation columnsare divided wall distillation columns. Such a column is a distillationvessel having a vertical partition separating one side from the otherfor a portion of the height of the vessel. Although such columncomprises a larger number of trays, the use of such single column may beadvantageous with regard to construction costs and energeticrequirements.

In a preferred embodiment, one or more of said distillation columns arepacking columns. Packing column refers to a column packed with inertsolid particles.

Reboilers are used as heat exchangers to provide heat to the bottom ofsaid distillation columns. They boil the liquid from the bottom of adistillation column to generate vapors which are returned to the columnto drive the distillation separation. The reboiler receives a liquidstream from the column bottom and may partially or completely vaporizethat stream. Steam usually provides the heat required for thevaporization. In accordance with the present invention, a portion of thebottom stream obtained in the distillation column is reboiled and saidre-boiled portion is returned to the distillation column.

In accordance with the present invention, separate streams of monomer,optionally co-monomer, and diluent are recycled for further use. Thevaporous feed stream to be separated, coming e.g. from the flash tanks,also comprises traces of both heavy components, e.g. oligomers, andlighter components including N₂, H₂, and light poisonous components suchas O₂, CO and CO₂, and formaldehyde. Such components are herein alsodenoted as “poisonous components”, because such components aredetrimental for the activity of a catalyst. Re-introduction thereof intoa polymerization reactor could greatly disturb catalyst activity andthus reduce polymerization efficiency. It is therefore of the utmostimportance to have a recovery system adapted to recover streams of (co-)monomer, and diluent, having residual amounts of such poisonouscomponents which are appropriate depending on their conditions of re-usein a polymerization process, e.g. depending on the reactor wherein theyare fed in a bimodal system.

In accordance with the present invention, different diluent-containingproduct streams are separated from the feed stream and can be re-used ina polymerization process for preparing bimodal polymer. More inparticular, in accordance with the present invention, saidhydrocarbon-containing feed stream is separated into

-   -   I) a bottom stream comprising substantially olefin-free        hydrocarbon diluent;    -   II) an overhead vapor stream comprising olefin monomer,        hydrocarbon diluent and components such as formaldehyde, H₂, N₂,        O₂, CO and CO₂;    -   III) a first side stream comprising hydrocarbon diluent and        olefin monomer; and    -   IV) a second side stream which is substantially hydrogen-free        and comprises hydrocarbon diluent and olefin monomer.

According to a particular embodiment of the present invention thehydrocarbon-containing feed stream is condensed before being separatedinto the product streams given above.

In another preferred embodiment said hydrocarbon-containing feed streamcomes as an overhead vapour stream from another (first) distillationcolumn, wherein the feed has been subjected to distillation conditionsadapted to remove a1) a bottom stream comprising hydrocarbon diluent andone or more optional co-monomers, and a2) an overhead stream comprisinghydrocarbon diluent, olefin monomer and components such as H₂, N₂, O₂,CO, CO₂, and formaldehyde.

More in particular, preferably, said overhead vapour stream obtained instep a2) is subjected to the following steps before being condensed bythe overhead condenser of a second distillation column:

-   -   the overhead stream issued from the first distillation column is        condensed to form a condensate and the thus formed condensate is        stored in a reflux drum (a first separator) of the first        distillation column; the reflux drum is adapted to separate a        vapor stream and a liquid stream;    -   the vapor stream is removed from said first separator,    -   the vapor stream is condensed to form a condensate and stored in        a second separator adapted to separate a vapor stream and a        liquid stream;    -   the liquid stream is removed from said second separator and this        liquid stream is separated into a first side stream and a        remainder stream;    -   the remainder stream is introduced in a second distillation        column and subjected to distillation conditions

In other words, from said reflux drum of the first distillation column(first separator) a portion of the stored condensate is removed asliquid stream and fed to the first distillation column; while anotherportion of the stored condensate is removed from the separator as vapourstream. This vapour stream comprises olefin monomer, hydrocarbon diluentand components such as formaldehyde, H₂, N₂, O₂, CO and CO₂. It is thelatter vapour stream that is then again condensed by the overheadcondenser of a second distillation column to form a condensate and thenstored in a reflux drum of the second distillation column before beingseparated into the product streams given above.

The bottom stream (I) comprises substantially olefin-free hydrocarbondiluent. The term “substantially olefin-free hydrocarbon diluent” or“olefin-free diluent” or the like are used herein as synonyms to denotehydrocarbon diluent which contains less than 5 ppm by weight, andpreferably less than 1 ppm by weight of monomer; and less than 5000 ppmby weight, preferably less than 1000 ppm by weight and more preferablyless than 100 ppm by weight of optional co-monomer. This bottom streamis also substantially free of hydrogen, and in particular contains onlytraces of hydrogen, for instance less than 10⁻² ppm, preferably lessthan 10⁻³ ppm of hydrogen. Substantially free of traces of monomer suchas ethylene and/or optional co-monomer such as hexene, and of lightercomponents such as hydrogen, the bottom stream of olefin freehydrocarbon diluent, such as isobutane, issued from the distillationcolumn can be sent to a storage tank and further used, e.g. for flushingconduits and circulation pumps in a polymerization reactor, or forcatalyst preparation e.g. in mud pots. This olefin-free diluent can alsobe recycled to a polymerization zone, whether homo-polymerization orco-polymerization, at any place of the process where pure diluent isrequested, like the catalyst dilution. When recycled in a bimodalpolymerization process according to the present invention, this bottomstream is fed to said reactor in which the polyolefin fraction havingthe higher molecular weight is prepared. In accordance with the presentprocess it is possible to reduce the amount of this bottom stream.

Light components such as formaldehyde, H₂, N₂, O₂, CO and CO₂ exit the(second) distillation column with some residual monomer and diluent asan overhead vapor stream (II). This overhead vapor exits at the top ofthe distillation column. The overhead stream is preferably condensed toform a condensate and is then stored. For instance this condensate ispassed to a separator, also denoted reflux drum or condensate vesselherein. A portion of this stored condensate is removed, e.g. it isremoved from a separator, as vapor stream comprising olefin monomer, andcomponents such as formaldehyde, H₂, N₂, O₂, CO and CO₂; and send to anMonomer Recovery Unit, such as for instance an ethylene recovery unit(ERU) in the event of using ethylene monomer. These light components arethen further treated in the Recovery Unit, which further separates thelight components from the remaining monomer and hydrocarbon diluent.Preferably, the amount of remaining monomer that is sent to a monomerrecovery unit is lower than 30%, preferably lower than 20%, preferablylower than 10%. Monomer and diluent that are recovered by means of therecovery unit are preferably re-used in the polymerization process.

In an example, under prior art conditions, the stream conveyed to anethylene recovery unit (ERU) comprises isobutane, ethylene, hydrogen,nitrogen and ethane. Ethylene and isobutane are further recovered inERU. Using the process of the invention allows reducing the amount ofethylene sent to the ERU, since ethylene monomer is recovered in anadditional side stream (IV) separated from the column. Preferably, theamount of remaining ethylene that is sent to the ERU is lower than 30%,preferably lower than 20%, preferably lower than 10%.

The side stream (III) of hydrocarbon diluent issued from thedistillation column is generally sent to a storage tank and furtherused. Preferably, the amount of further components such as H₂, N₂, O₂,CO and CO₂, formaldehyde in the side stream is preferably lower than 500ppm, and for instance comprised between 50 and 500 ppm. In anotherpreferred embodiment, the amount of monomer remaining in the side streamis lower than 10 wt %, for instance comprised between 5 and 10 wt %.High amounts of monomer in the storage tank of the side-stream productmay lead to evaporation and substantial monomer loss. By keeping theamount of monomer in the side-stream product below 10%, evaporation ofmonomer from the storage tank can be reduced and storage of theside-stream product at atmospheric conditions becomes possible.

The hydrocarbon diluent issued as side stream exiting from thedistillation column is in accordance with the present invention recycledand used as diluent in a polymerization process for preparing bimodalpolyolefin, and preferably it can be fed to that polymerization reactorin which the polyolefin fraction with the lower molecular weight isprepared, e.g. a second reactor.

Also the side stream (IV) of hydrocarbon diluent issued from thedistillation column is generally sent to a storage tank and furtherused. This second side stream is substantially hydrogen-free andcomprises hydrocarbon diluent and olefin monomer. The term“substantially hydrogen-free” or “substantially hydrogen-freehydrocarbon diluent” or the like are used herein as synonyms to denotehydrocarbon diluent which contains less than 5 ppm by weight, andpreferably less than 1 ppm by weight, and even more preferred less than0.5 ppm by weight hydrogen. In another preferred embodiment, the amountof monomer remaining in the second side stream is lower than 5 wt %, forinstance between 2 to 5 wt %.

The hydrocarbon diluent issued as second side stream exiting from thedistillation column is in accordance with the present invention recycledand used as diluent in a polymerization process for preparing bimodalpolyolefin, and preferably it can be fed to that polymerization reactorin which the polyolefin fraction with the higher molecular weight isprepared, e.g. a first reactor.

In a preferred embodiment, a process is provided wherein said secondside stream is removed from the upper third of said second distillationcolumn. The Applicants have shown that removing a side stream from thispart of the column provides a side stream has the most optimalcomposition for being recycled in a reactor in which the polyolefinfraction with the higher molecular weight is prepared, e.g. a firstreactor. In another embodiment, the present process is carried out in adistillation system, which comprises one or more tray distillationcolumns. In an example, in the case of using a distillation columnhaving 20 trays, the process provides that the second side stream isremoved from a tray of said distillation column which is selected fromthe upper 2^(nd) to 7^(th) tray, preferably from the upper 3^(rd) and6^(th) tray in said distillation column. With “upper” in this context ismeant as counted from the top of said distillation column.

In one embodiment, the present process is carried out in a distillationsystem, which comprises two distillation columns.

In yet another embodiment, the present process is carried out in adistillation system, which comprises three distillation columns.

Referring now to the figures, a recycling unit according to anembodiment of the invention is schematically illustrated in FIG. 1. Theillustrated recycling section is composed of two distillation columns 1,2 and of an monomer recovery unit represented by the box 19. Thedistillation columns preferably are tray columns. Thehydrocarbon-containing feed stream 4 that will be separated willgenerally be an overhead stream coming from a flash tank and purgecolumns of a polymerization reactor (not shown). A first distillationcolumn 1 realizes a rough cut between a mixture of diluent such asisobutane, comonomer such as hexene and the heavies, which exit asliquid bottom product 5. The heavy bottom product 5 can be furthertreated (not shown). The remaining monomer, together with some diluentand all light components, exits from the top of the first distillationcolumn 1 as a vapor stream 6 for further separation. This overheadstream 6 from the first distillation column 1 is cooled down in anoverhead condenser 107. The stream 7 at the outlet of the condenser iscollected in a reflux drum 108. The reflux drum 108 is adapted toseparate the stream 7 into a liquid stream 8 and a vapor stream 106. Theliquid stream 8 from the reflux drum is used as reflux to the firstdistillation column 1 and the vapor stream 106 obtained at the outlet ofthe reflux drum 108 is sent to a second distillation column. However,this vapor stream 106 is not directly fed to the second distillationcolumn as such but it is first condensed in an overhead condenser 207.The overhead condenser 207 of the second distillation column 2 thusreceives the vapor stream 106 from the reflux drum 108 of the firstdistillation column 1 and optionally also an overhead stream 12 of thesecond distillation column (to be described later). Thus an overheadstream 12 to be described later is combined with stream 106 for passagethrough a condenser 207 and introduction in a separator 208. The stream13 at the outlet of the condenser 207 of the second distillation columnis collected in a reflux drum 208, preferably different from the refluxdrum of the first distillation column described above. This reflux drum208 is adapted to separate a liquid stream 15 and a vapor stream 14. Theliquid stream is removed from the reflux drum 208 of the seconddistillation column 2 and is then split (separated) into a first sidestream 9 comprising hydrocarbon diluent and olefin monomer; and aremainder stream 16. The remainder stream 16 is used as reflux to thesecond distillation column 2. In other words, a part of the liquidcondensate 15 comprising monomer, diluent and lighter components such asformaldehyde, N₂, H₂, and components such as O₂, CO and CO₂ is passedfrom the condensate vessel 208 as feed to the second distillation column2.

Diluent, lighter components such as H₂ and residual amounts of monomer,are removed as a liquid side stream 9 from the condensate vessel 208.This side stream 9 can be introduced into a storage vessel for storageand further handling. Preferably this side stream comprises less than 10wt % monomer and less than 500 ppm by weight hydrogen. This first sidestream 9 is usually recycled in a polymerization reactor. Preferablywhen this stream 9 is recycled in a polymerization process for preparingbimodal polyolefin comprising at least two different polyolefinfractions that have been obtained in two different polymerisationreactors connected to each other in series, and wherein one of saidfractions has a higher molecular weight than said other fraction, it isre-used in the polymerization process wherein the other polyolefinfraction is prepared. The vapor stream 14 separated from the reflux drum208 of the second distillation column 2 is further cooled down in a ventcondenser (not shown), before being sent to a monomer recovery unit 19where monomer is recovered and lights components such as H₂ and N₂ aresent to the flare.

Conditions within the distillation column 2 are such that differentproduct streams will be generated. Substantially pure diluent, so-called“substantially olefin-free” diluent is obtained as liquid bottom product11. This bottom stream 11, which comprises diluent substantially free ofolefin monomer, is removed from a lower portion of the distillationcolumn 2. This bottom stream can be passed through a water-cooled heatexchanger (not shown), and introduced into vessel (not shown) forstorage and further handling. The second distillation column 2 furthercomprises devices for providing re-boiling of a portion of the bottomstream 11 under a controlled steam flow rate. For instance, according tothe invention a portion of bottom stream 11 is passed through asteam-heated heat exchanger 18 and returned to the distillation columnby way of line 17. Remaining diluent exits the column 2 as a secondliquid side stream 10, which is substantially hydrogen-free. Accordingto a preferred embodiment, this side stream 10 is removed from an upperthird part of the distillation column 2 by way of a line (not shown) andintroduced into vessel (not shown) for storage and further handling,e.g. from the 3^(rd), 4^(th), 5^(th) or 6^(th) upper tray in saiddistillation column 2. Light components such as formaldehyde, H₂, N₂,O₂, CO and CO₂ exit the distillation column 2 with olefin monomer andsome residual diluent as a vapor stream 12, which according to anotherembodiment of the invention can be further condensed by a condenser 207.The condensed vapor stream 13 is then sent to the separator 208.

An incondensable vapor stream 14 is separated from the separator 208,passed through a vent condenser (not shown) for recovery of most of theentrained diluent, and then further treated in an olefin monomerrecovery unit 19.

The distillation process in the second distillation column thus permitsto separate substantially olefin-free diluent in a bottom stream 11 aswell as diluent containing residual amounts of olefin monomer and a lowamount of hydrogen, e.g. less than 5 ppm, in a side stream 10 and alsodiluent containing residual amounts of olefin monomer and some higheramounts of hydrogen in a side stream 9. Both the substantiallyolefin-free diluent 11 and the diluent 10 can be recycled and re-used ina polymerization process for preparing bimodal polyolefin, in particularin the reactor in which the polyolefin fraction having the highermolecular weight is prepared. Also the diluent 9 can be recycled andre-used in a polymerization process of preparing bimodal polyolefin, inparticular in the reactor in which the polyolefin fraction having thelower molecular weight is prepared. In a particularly preferredembodiment, the monomer recovery unit 19 can be avoided. The separationof side stream 10 comprising monomer and diluent decreases dramaticallythe stream 14, leading to a competitive process in terms of monomerrecovery without the need of having a monomer recovery unit.

As illustrated in FIG. 2, a recycling unit according to an embodiment ofthe invention can be composed of three distillation columns 1, 2, 3, inaddition to a monomer recovery unit represented by the box 19. Thehydrocarbon-containing feed stream 4 that is to be separated willgenerally be an overhead stream coming from a flash tank and purgecolumns of a polymerization reactor, wherein a stream containingsolvent, polymer and unreacted monomers is flashed or otherwise treatedto remove solvent or diluent and monomers therefrom. A firstdistillation column 1 realizes a rough cut between a mixture of diluent,co-monomer and the heavies, which exit as liquid bottom product 5. Theheavy bottom product is further treated in a distillation column 3 andseparated into three product streams: diluent vapor exiting as topproduct 22 is first cooled down in the overhead condenser 307 of thethird distillation column. Then the stream 23 at the outlet of thiscondenser 307 is collected in a reflux drum 308 of the thirddistillation column 3. This reflux drum 308 is adapted to separate andcollect the fully condensed stream 23. This liquid then exits the refluxdrum 308 as liquid stream 24. The liquid stream 24 can be further splitinto a first liquid stream 26 which is sent as reflux to the thirddistillation column and a second liquid stream 25 which can be recycledto the first distillation column.

A purified liquid comonomer stream 21 is recovered from a tray justabove the column sump and sent to storage for recycling to thepolymerization reactor(s). The heavy components 20 are recovered fromthe column sump with the draining procedure being triggered on highcolumn bottoms temperature.

The remaining monomer, diluent, with all light components, which exitsfrom the top 6 of the first distillation column 1, is sent to adistillation column 2 as a vapor stream for further separation accordingto the process as described for FIG. 1. The stream will be condensed andseparated at least twice in a condenser/separation cycle prior toentering said distillation second column 2 and treated in a same way asexplained for FIG. 1. The distillation column 2 is used to generate fourproduct streams: substantially pure diluent, so-called “substantiallyolefin-free” diluent is obtained as liquid bottom product 11. Lightcomponents such as formaldehyde, H₂, N₂, O₂, CO and CO₂ exit thedistillation column 2 with olefin monomer and some residual diluent as avapor stream 12, which according to an embodiment of the invention arefurther condensed before being further purified and separated in amonomer recovery unit represented by the box 19. Remaining diluent exitsthe column 2 as a liquid side stream 10. In addition, a separated sidestream of diluent and monomer 9 is also recycled and re-used in apolymerization process.

This invention can be further illustrated by the following example of apreferred embodiment of the invention, although it will be understoodthat this example is included merely for purposes of illustration andare not intended to limit the scope of the invention unless otherwisespecifically indicated.

Example

This example illustrates the separation of a hydrocarbon-containing feedstream according to a process of the invention into a) a first sidestream comprising isobutane and ethylene; b) a second side stream whichis substantially hydrogen-free and comprises isobutane and ethylene, c)a bottom stream comprising substantially ethylene-free isobutane, and d)an overhead vapor stream comprising isobutane and ethylene andcomponents such as formaldehyde, H₂, N₂, O₂, CO and CO₂. A recyclingsection according to an embodiment of the invention wherein such processis carried out is illustrated in FIG. 1. This recycling section setupcan be used to recover a large part or most of the isobutane withminimized losses of ethylene.

In this example, the second side stream is drawn from the 6^(th) tray(counted from the top) of the second column. In such case it wascalculated that a reboiler steam consumption of 800 kg/hour would berequired and that the resulting total flow rate sent to the ERU wouldamount to some 325 kg/hour. These calculated values can be compared toconditions applied in a recycle system wherein no second side stream isseparated, which means that line 10 is then absent and this flow ratethen adds to the one exiting the column through line 11. In that latercase steam flow rate reaches 1500 kg/hour and the flow rate to the ERUis above 700 kg/hour.

These values illustrate that separating a substantially hydrogen-freesecond side stream advantageously permits to strongly reduce the netrequirement for ethylene-free isobutane (bottom stream). This makes itpossible to reduce the steam consumption on the (second) distillationcolumn, even by more than 700 kg/hour. In addition, this second sidestream will take up some ethylene and will significantly reduce,preferably by more than 50%, the incondensable vapor stream stillcontaining hydrogen that is removed from the separator of the seconddistillation column. Hence the loss of ethylene will be substantiallyreduced.

Table 1 given below compares the composition of a second side stream(nr. 10 in FIG. 1), separated in accordance with the present process,and as compared to a conventional side stream issued in a conventionalrecycling system.

TABLE 1 Side stream according Conventional to the invention side streamHydrogen (ppm) 0.33 318.54 Ethylene (% wt) 2.83 7.28 Isobutane (% wt)97.16 92.65 Hexene (ppm) 0.02 0.02

As illustrated in Table 1, having an additional first side stream (9)permits to separate a second side stream (10) from an upper part of thedistillation column comprising isobutane and ethylene, but which issubstantially free of hydrogen. For comparison, in a conventionalsystem, a side stream separated from a distillation column containsapproximately similar amounts of isobutane and ethylene but containsmuch more hydrogen.

1. Process for recycling product streams separated from ahydrocarbon-containing feed stream comprising olefin monomer, one ormore optional olefin co-monomer, hydrocarbon diluent and components suchas H₂, N₂, O₂, CO, CO₂, and formaldehyde, wherein saidhydrocarbon-containing feed stream is separated by the steps of: a)introducing said feed stream into a first distillation column (1) forsubjecting said feed to distillation conditions adapted to remove a1) abottom stream (5) comprising hydrocarbon diluent and one or moreoptional co-monomer, and a2) an overhead stream (6) comprisinghydrocarbon diluent, olefin monomer and components such as H₂, N₂, O₂,CO, CO₂, and formaldehyde; b) condensing the overhead stream (6) issuedfrom the first distillation column (1) in step a2) to form a condensateand storing said condensate in a separator (108) adapted to separate avapor stream (106) and a liquid stream (8); c) removing from saidseparator (108) said vapor stream (106) comprising olefin monomer,hydrocarbon diluent and components such as formaldehyde, H₂, N₂, O₂, COand CO₂; d) condensing the vapor stream (106) removed in step e) to forma condensate (13) and storing said condensate in a separator (208)adapted to separate a vapor stream (14) and a liquid stream (15); e)removing from said separator (208) said liquid stream (15) of step d);f) separating said liquid stream (15) into a first side stream (9)comprising hydrocarbon diluent and olefin monomer; and a remainderstream (16); g) introducing said remainder stream (16) in a seconddistillation column (2) and subjecting said remainder stream (16) todistillation conditions adapted to remove g1) a bottom stream (11)comprising substantially olefin-free hydrocarbon diluent, g2) asubstantially hydrogen-free second side stream (10) comprisinghydrocarbon diluent and olefin monomer, and g3) an overhead vapor stream(12) comprising olefin monomer, hydrocarbon diluent and components suchas formaldehyde, H₂, N₂, O₂, CO and CO₂, furthermore wherein said first(9) and said second (10) side streams are recycled in a polymerizationprocess for preparing bimodal polyolefin comprising at least twodifferent polyolefin fractions that have been obtained in two differentpolymerisation reactors connected to each other in series, and whereinone of said fractions has a higher molecular weight than said otherfraction, and wherein said second side stream (10) is re-used in thepolymerization process wherein the polyolefin fraction having the highermolecular weight is prepared, and wherein said first side stream (9) isre-used in the polymerization process wherein the other polyolefinfraction is prepared.
 2. Process according to claim 1, comprising thesteps of h) condensing the overhead vapor stream (12) obtained in stepg3), optionally in admixture of the vapor stream (106) removed in stepc) to form a condensate, and storing the condensate thus formed in aseparator (208); and i) subjecting the stored condensate obtained instep h) to steps e) to g).
 3. Process according to claim 1, wherein saidsecond side stream of step g2) (10) comprises at least 2 wt % olefinmonomer, and preferably at least 3 wt % olefin monomer.
 4. Processaccording to claim 1, wherein said second side stream of step g2) (10)comprises at most 5 ppm by weight hydrogen, and preferably at most 1 ppmby weight hydrogen.
 5. Process according to claim 1, wherein said firstside stream of step f) (9) comprises less than 10 wt % olefin monomer.6. Process according to claim 1, wherein said first side stream of stepf) (9) comprises less than 500 ppm by weight hydrogen.
 7. Processaccording to claim 1, wherein said bottom stream of step g1) (11)comprises less than 5 ppm of olefin monomer and optionally less than5000 ppm of olefin co-monomer.
 8. Process according to claim 1, whereinsaid second side stream of step g2) (10) is removed from the upper thirdof said second distillation column.
 9. Process according to claim 1,wherein said bottom stream of step g1) (11) is re-used in apolymerization process for preparing bimodal polyolefin comprising atleast two different polyolefin fractions that have been obtained in twodifferent polymerisation reactors connected to each other in series, andwherein one of said fractions has a higher molecular weight than saidother fraction, and wherein said bottom stream (11) is re-used in thepolymerization process wherein the polyolefin fraction having the highermolecular weight is prepared.
 10. Process according to claim 1,comprising the step of removing from the condensate stored in step d) avapor stream (14) comprising olefin monomer, and components such asformaldehyde, H₂, N₂, O₂, CO and CO₂; and recovering olefin monomer fromsaid vapor stream (14).
 11. Process according to claim 1, comprisingintroducing the bottom stream (5) of step a1) in a third distillationcolumn (3) for subjecting said bottom stream (5) to distillationconditions adapted to remove 1) a side stream (21) comprising one ormore optional co-monomer, 2) an overhead stream (22) comprisinghydrocarbon diluent and optionally co-monomer, and 3) a bottom stream(20) comprising heavy components.
 12. Process according to claim 1,wherein said hydrocarbon-containing feed stream (4) comprising olefinmonomer, one or more optional olefin co-monomer and hydrocarbon diluentis an effluent stream obtained from a polymerization process forpreparing monomodal or bimodal polyolefin.
 13. Polymerization system forpreparing bimodal polyolefin comprising two different polymerisationreactors connected to each other in series operably connected to adistillation system, the distillation system comprising a firstdistillation column (1) and a second distillation column (2) which areoperably connected to each other in series; and wherein said firstdistillation column (1) which is configured to separate ahydrocarbon-containing feed stream (4) comprising olefin monomer,optionally one or more co-monomer and hydrocarbon diluent, into a bottomstream (5) comprising hydrocarbon diluent and one or more optionalco-monomer, and an overhead stream (6) comprising hydrocarbon diluent,olefin monomer and components such as H₂, N₂, O₂, CO, CO₂, andformaldehyde; is provided with at least one condenser (107) forcondensing said overhead feed stream (6) to form a condensed stream (7)and at least one separator (108), operably connected to said condenserand adapted to separate said condensed stream (7) in a vapor stream(106) and a liquid stream (8); and wherein said second distillationcolumn (2) which is configured to separate a hydrocarbon-containing feedstream (106) comprising olefin monomer, optionally one or moreco-monomer and hydrocarbon diluent, into a bottom stream (11) comprisingsubstantially olefin-free hydrocarbon diluent, a substantiallyhydrogen-free side stream (10) comprising hydrocarbon diluent and olefinmonomer, and an overhead stream (12) comprising olefin monomer,hydrocarbon diluent and components such as formaldehyde, H₂, N₂, O₂, COand CO₂, is provided with at least one condenser (207) for condensingsaid overhead stream (12) to form a condensed stream (13) and at leastone separator (208), operably connected to said condenser (207) andadapted to separate said condensed stream (13) in a vapor stream (14)and a liquid stream (15), of which a part is separated as a first sidestream (9) comprising hydrocarbon diluent and olefin monomer, which isrecycled in a polymerization reactor.
 14. Use of a process according toclaim 1 in a polymerization process for preparing bimodal polyolefincomprising at least two different polyolefin fractions that have beenobtained in two different polymerisation reactors connected to eachother in series, and wherein one of said fractions has a highermolecular weight, comprising the steps of: feeding olefin monomer, adiluent, at least one polymerization catalyst, optionally hydrogen, andone or more optional olefin co-monomer(s) to a first reactor,polymerizing said olefin monomer in said first reactor to produce apolymer slurry comprising a first polyolefin fraction in the diluent,transferring said polymer slurry from said first reactor to a secondreactor, feeding olefin monomer, a diluent, optionally hydrogen, and oneor more optional olefin co-monomer(s) to said second reactor,polymerizing said olefin monomer and said one or more optional olefinco-monomer(s) in said second reactor to produce a slurry comprising asecond polyolefin fraction in the diluent, said second polyolefinfraction having a different molecular weight than the polyolefinfraction produced in said first reactor, and discharging from saidsecond reactor a slurry comprising bimodal polyolefin in said diluent,recovering bimodal polyolefin from the slurry by separating at least amajority of the diluent from the slurry in a hydrocarbon-containing feedstream, and subjecting said hydrocarbon-containing feed stream to aprocess according to claim 1.